The continuous removal of fine solid particles of density similar to the carrying liquid is very difficult and expensive operation. The most effective method is based on applying very high centrifugal forces to the liquid with suspended solid particles in stacked conical disc centrifuges, which forces effectively cause the denser particles to settle on the disc surface and slide outward whereas the clarified liquid flows inward under an applied hydraulic pressure. These devices are very expensive and are impractical to use for low capital budget projects. A less capital intensive approach is to use two or more filter bags with appropriate mesh size cloth and remove them in sequence to empty them of collected solids. The filter bag approach is obviously labor intensive and therefore expensive in operation. Yet another method is to filter liquid through a screen under high centrifugal forces and periodically or continuously clean the deposits from the screen by some mechanical means or forces. The net forces on liquids and solids are in the same direction and as a result the filtration surface has a tendency to be only partially open for filtration at any time. A large filtration surface area is therefore required which makes the equipment quite expensive. Also, some particles are forced through the openings in the filtration surface. In yet another method, high hydraulic forces derived from a pump drive the liquid in a circular motion in a cylindrical-conical container, called hydro-cyclone, and the resulting centrifugal forces cause the solid and liquids of different densities to separate radially. However, unless the density difference between solid particles and carrying liquid is significant and/or the particles size is large, this method does not provide reasonable level of clarification.
A method for clarification of the invention disclosed here yields an equipment that is low in the initial cost as well as in operating cost. The method uses filtration to remove fine particles from the liquid while using centrifugal forces, in the reverse direction to the hydraulic forces derived from a pump, to drive the particles away from the screen in order to continuously maintain the filtration surface clean of any solid particles. An apparatus of the invention to carry out the method described above includes a centrifugal screen enclosure provided in a vessel. The clarified liquid comes out from a first outlet and a liquid/solid sludge from a second outlet.